Some Performance Advice Needed

Hi,
i recently run pgbench against different servers and got some results I
dont quite understand.

A) EV1: Dual Xenon, 2GHz, 1GB Memory, SCSI 10Krpm, RHE3
B) Dual Pentium3 1.4ghz (Blade), SCSI Disk 10Krmp, 1GB Memory, Redhat 8
C) P4 3.2GHz, IDE 7.2Krpm, 1GBMem, Fedora Core2

All did run only postgres 7.4.6

pgconf settings:
max_connections = 100
shared_buffers = 8192
sort_mem = 8192
vacuum_mem = 32768
max_fsm_pages = 200000
max_fsm_relations = 10000
wal_sync_method = fsync
wal_buffers = 64
checkpoint_segments = 10
effective_cache_size = 65536
random_page_cost = 1.4

/etc/sysctl.conf
shmall and shmmax set to 768mb

Runnig PGbench reported
A) 220 tps
B) 240 tps
C) 510 tps

Running hdparm reported
A) 920mb/s (SCSI 10k)
B) 270mb/s (SCSI 10k)
C) 1750mb/s (IDE 7.2k)

What I dont quite understand is why a P3.2 is twice as fast as a Dual
Xenon with SCSI disks, A dual Xenon 2GHz is not faster than a dual P3
1.4Ghz, and the hdparm results also dont make much sense.

Has anybody an explanation for that? Is there something I can do to get
more performance out of the SCSI disks?

Thanks for any advise
Alex